Obama warns South Sudan after US military aircraft attacked

President Barack Obama was briefed by his national security team on Saturday after four US military service members were wounded when their aircraft were fired upon during a mission to evacuate American citizens from chaotic South Sudan.

"Any effort to seize power through the use of military force will result in the end of longstanding support from the United States and the international community," the White House said in a statement following Obama's call with National Security Adviser Susan Rice and other top aides.

The president, who is vacationing in Hawaii, directed his team to ensure the safety of US military personnel, and to continue to work with the United Nations to evacuate American citizens from Bor, which is caught up in the spiraling conflict in South Sudan.

After the call, Obama "underscored that South Sudan's leaders have a responsibility to support our efforts to secure American personnel and citizens in Juba (South Sudan's capital) and Bor," the White House said.

The US aircraft - three Osprey CV-22s - came under fire while approaching the evacuation site, the US military's Africa Command said in a statement.

"The aircraft diverted to an airfield outside the country and aborted the mission," it added.

On Thursday, Obama said that South Sudan, which achieved independence in 2011 to become Africa's newest nation, "stands at the precipice."

Earlier in the week, Obama deployed 45 US service members to the land-locked country "to support the security of US personnel and our Embassy."